Birds carry a fungus that can be fatal in people, yet the birds don’t get sick. Now researchers think they know why.

Cryptococcus neoformans is a fungus that causes deadly infections in people with a weakened immune system. It’s found in bird droppings, and it’s one of the most dangerous infections for people with AIDS—and is thought to cause hundreds of thousands of deaths worldwide every year.

New research shows that a particular white blood cell within the bird’s blood system, called a macrophage, is able to completely block the growth of Cryptococci.

The fungus can grow slowly within the bird’s digestive tract, but if it tries to invade the bird’s body then the immune system immediately destroys it—which explains why healthy birds can still help spread the infection.

“Understanding where the disease comes from and how it spreads is critical. If we can learn how some animals are able to resist infection we might be able to gain insights into how we can improve the human immune response to this fungus,” says team leader Simon Johnston from the University of Sheffield.

This work, published in Nature Scientific Reports, was carried out in collaboration with the University of Birmingham and is part of a much larger international effort to understand, fight, and ultimately eliminate cryptococcosis.

“We are now working with leading scientists from all over the world to try and understand where this pathogen came from, how our bodies fight it, and what we can do to help our own immune system defend us from this fungus and other related infections.

“Many human diseases are spread by birds, but we know surprisingly little about their immune systems. Discovering how they resist otherwise fatal infections offers the hope of improving our ability to intervene in this cycle and prevent a diverse range of human diseases.”

The Medical Research Council, Lister institute for Preventative Medicine, and Wellcome Trust supported the project.